In the manufacture of pipe that has a welded seam, it is common to use multiple AC welding arcs at extremely high current levels, such as over 1,000–2,000 amperes. The less expensive power supply to create such ultra high welding currents is a transformer based welder having a sinusoidal output current. This power supply requires only a large, heavy transformer and related control circuitry. However, to accomplish high welding currents the sinusoidal output has an extremely high peak current compared to the heating current determined by the root mean square of the sinusoidal wave. This relatively inexpensive power supply can create the necessary high current, but results in peak currents that seriously affect the welding operation. To overcome the disadvantages of a sinusoidal type electric arc welder, it is now common practice to use power supplies based upon high frequency switching technology. These switching type power supplies rectify the incoming line voltage to produce a DC link. This DC link is switched through a primary winding of an output transformer as alternating pulses to create an output current constituting the AC arc welding current. Pulse width modulators determine the frequency in the primary winding of the output transformer. Consequently, the pulses at the output transformer are substantially square waves. Thus, the root mean square of the secondary current is essentially the same as the maximum output current for the power supply. In this manner, welding arc does not require high peak currents to obtain the desired root mean square current for heating. Consequently, the inverter type power supply overcomes the disadvantage of the sinusoidal power supply when performing high current electric arc welding of the type needed for seam welding pipes. For this reason, pipe welding has been converted to the inverter technology.
Even though widely used for pipe welding, inverters present a dilemma. Standard inverter type power supplies generally have a maximum output in the range of 500 amperes. To provide an inverter type power supply for high currents in excess of 1,000–2,000 amperes, a special inverter must be designed and engineered. This involves substantial costs and highly trained electrical and welding engineers. But, such high capacity power supply has a relatively low sales volume. Consequently, high current inverters for use in pipe welding are not economically feasible and demand a long lead time. To overcome these disadvantages, The Lincoln Electric Company has developed a power supply using a master inverter, with one or more slave inverters controlled and operated in unison. When the welding operation requires a current in excess of 1500 amperes, three inverters are parallel. The rated output current for the compound inverter is tripled over a single off-the-shelf inverter. Increasing the number of inverters operated in unison to provide a high current type welder is expensive, but accomplishes the desired results.
There is a need for a high current power supply that creates an AC welding current having a root mean square current of over 1,000–2,000 amperes without the requirement of paralleling several standard low current inverters. Such high current power supply for use in electric arc welding of pipes must not have the peak current problem, experienced by a sinusoidal type power supply.